Presbyopia is the impairment of vision due to advancing years or old age. Presbyopia is what causes a middle-aged or older person to hold a newspaper, magazine, or book at arm's length to read it. Many presbyopic individuals wear bifocals to help them cope with presbyopia. Presbyopia is typically observed in individuals over 40 years of age. Presbyopic individuals suffering from presbyopia may have normal vision, but the ability to focus on near objects is at least partially lost over time, and those individuals come to need glasses for tasks requiring near vision, such as reading. Presbyopia affects almost all individuals over the age of 40 to a greater or lesser degree.
For an eye to produce a clear image of objects at different distances, the effective focal length of the eye is adjusted to keep the image of the object focused on the retina at the back of the eye. Accommodation refers to this change in effective focal length. Accommodation is the ability of the eye to change its focus and is accomplished primarily by varying the shape of the crystalline lens. Accommodation provides the ability to change focus from distant objects to near objects. The ability to change focus from distant objects to near objects is impacted by presbyopia.
The shape of the crystalline lens is varied by the use of certain muscles and structures within the eye. As shown in FIG. 1, the crystalline lens 114 is located in the forward part of the eye. The crystalline lens has a generally circular cross-section having two convex refracting surfaces. The curvature of the posterior surface of the lens (which is nearer to the vitreous body) is greater than that of the anterior surface. The crystalline lens is suspended by a circular assembly of collagenous fibers called zonules 104, which are attached at their inner ends to the lens capsule (the outer surface of the crystalline lens) and at their outer ends to the ciliary body 115, a muscular ring of tissue located just within the outer supporting structure of the eye, the sclera 101. The ciliary body 115 is relaxed in the unaccommodated eye and therefore assumes its largest diameter.
During an individual's life, the crystalline lens continues to grow by epithelial cell division at the equator of the crystalline lens and formation of differentiated fiber cells from some epithelial cells. Presbyopia is generally believed to occur at least partially because of continued growth of the crystalline lens. One result of such growth is a progressive reduction in the flexibility of the crystalline lens, thus leading to the continuous decrease of accommodation. The growth in size of the lens in the confines of the capsule cause it to lose its ability to focus.
Previous approaches to treating presbyopia have been addressed to the cornea or sclera of the eye, although there have been suggestions to treat presbyopia by addressing the crystalline lens. One technique, called photophako reduction (PPR), would employ a laser to create cavities in the lens, thereby reducing its size. Another technique, called photophako modulation (PPM) would employ a laser to create minute perforations in the lens to soften it. Another technique involves using a photodisruptive laser to soften the inside of the crystalline lens to restore elasticity.
Other attempts to treat presbyopia have involved addressing the sclera or zonules. Laser Presbyopia Reversal (LAPR) involves using infrared lasers for ablation of parts of the sclera. Surgeons use the lasers to make spoke-like excisions in the sclera to thin it and give the lens more room to function. Another approach called Anterior Ciliary Sclerotomy (ACS) has attempted to make the fibers attached to the lens taut by placing several partial thickness incisions on the sclera or white part of the eye in a radial pattern. Some surgeons have placed silicone implants inside the radial incisions, trying to prevent the regression. Yet another technique for tightening the lens fibers involves applying an infrared laser to strategically thin the sclera.
U.S. Pat. No. 5,465,737 (Schachar) and other patents issued to the same inventor describe treating presbyopia and hyperopia by a method which increases the amplitude of accommodation by increasing the effective working distance of the ciliary muscle in the presbyopic eye. Schachar states that presbyopia is also arrested by inhibiting the continued growth of the crystalline lens by application of heat, radiation or antimitotic drugs to the epithelium of the lens.
Other techniques for treating presbyopia have been suggested, the most common being addressed to the cornea or the sclera. U.S. Pat. No. 6,258,082 describes a method and surgical technique for corneal reshaping and for presbyopia correction. The preferred embodiments of the system consists of a scanner, a beam spot controller and coupling fibers and a basic laser. Presbyopia is treated by a method which uses ablative laser to ablate the sclera tissue and increase the accommodation of the ciliary body.
U.S. Pat. No. 6,263,879 describes treating presbyopia by a method which uses ablative lasers to ablate the sclera tissue and increase the accommodation of the ciliary body. A scanning system is proposed to perform various patterns on the sclera area of the cornea to treat presbyopia and to prevent other eye disorder such as glaucoma.
U.S. Pat. No. 6,491,688 describes a method and apparatus for presbyopia correction. The disclosed preferred embodiments of the system consists of a beam spot controller, a beam delivery device, a slit lamp, a visible aiming beam and a selected solid state laser. Presbyopia is treated by the thermal contraction of the human zonnulas with a temperature increase of about (15-50) degree-C. generated by the selected lasers. The near infrared laser is focused and delivered by a gonio lens to the target zonnulas area and viewed by a surgeon using a slip lamp. The selected laser having optimal absorption characteristics is tightly focused such that only the target zonnulas is heated, while the cornea, the lens body and the adjacent areas are not damaged.
U.S. Pat. No. 6,663,619 (VISX Incorporated) discloses an ophthalmic surgery system and method for treating presbyopia by performing ablative photodecomposition of the corneal surface. A laser system ablates tissue to a predetermined ablation shape, and the cornea heals significantly to form a multifocal shape correcting presbyopia. The multifocal shape corrects for near-vision centrally and far-vision peripherally. The system and method enables wide area treatment with a laser having a narrower beam than the treatment area, and can be used in the treatment of many conditions in conjunction with presbyopia such as hyperopia, hyperopic astigmatism and irregular refractive aberrations.
U.S. Pat. No. 6,745,775 (Surgilight, Inc.) describes treating presbyopia by a method which uses various lasers to remove a portion of the scleral tissue and increase the accommodation of the presbyopic patient's eye.
U.S. Pat. Nos. 5,312,320 (VISX, Incorporated) describes controlled ablation of the cornea, using ultraviolet laser radiation, wherein irradiated flux density and exposure time are so controlled as to achieve desired depth of the ablation. Sculpturing action results from precharacterized distribution of flux density across the cross-section of laser-beam projection, in the context of beam size, at cornea incidence, to match the area to be ablated, and the duration of exposure determines the extent of curvature change. Illustrative techniques and situations are disclosed, for myopia correction, for hyperopia correction, and for astigmatism correction.
U.S. Pat. Nos. 5,711,762 and 5,735,843 (VISX, Incorporated) describes an argon-fluoride excimer laser or other laser source that directs its radiation through a mask and onto corneal tissue, or other biological matter, to form an ablation therein of predetermined configuration and depth by a process of ablative photodecomposition. The masks are formed with a slit, circular, crescent or other openings of widths between 30 and 800 microns, and may even be formed to provide a graded intensity center to edge. The mask is reflective or composed of or faced with an organic polymer to prevent heat build-up.
U.S. Pat. No. 6,325,792 (Swinger and Lai) describes the application of low energy, ultra-short (femptosecond) pulsed laser radiation to the patient's eye in one of a number of patterns such that the exposed ocular tissue is ablated or excised through the process of optical breakdown or photodisruption in a very controlled fashion. Using the laser inside the eye allows the surgeon to perform glaucoma operations such as trabeculoplasty and iridotomy, cataract techniques such as capsulectomy, capsulorhexis and phacoablation, and vitreoretinal surgery, such as membrane resection. The various procedures are accomplished by controlling energy flux or irradiance, geometric deposition of beam exposure and exposure time.
U.S. Pat. No. 6,706,036 (Lai) describes a laser-based method and apparatus for corneal surgery. The method and apparatus are intended to be applied primarily to ablate organic materials, and human cornea in particular. The method and apparatus uses a laser source which has the characteristics of providing a shallow ablation depth (0.2 microns or less per laser pulse), and a low ablation energy density threshold (less than or equal to about 10 mJ/cm2), to achieve optically smooth ablated corneal surfaces. Lai states that the surgical system can be used to perform surgical procedures including removal of corneal scar, making incisions, cornea transplants, and to correct myopia, hyperopia, astigmatism, and other corneal surface profile defects.
U.S. Pat. No. 5,439,462 (Intelligent Surgical Lasers) describes an ophthalmic laser system removing cataractous tissue from the lens capsule of an eye by phacofragmentation of the lens tissue for subsequent aspiration of the treated tissue.